Internal combustion engines, which use homogeneous-charge compression ignition (HCCl) in addition to spark ignition (SI), have been proposed. HCCl is a form of internal combustion in which a homogeneous charge of air-fuel mixture is compressed in a combustion chamber to start ignition, and SI is a form of internal combustion in which combustion begins when air-fuel mixture in a combustion chamber is ignited by a spark. In HCCl, combustion occurs at a lower temperature in comparison to spark ignition, resulting in low Nitrogen oxide (NOx) emissions. HCCl has another characteristic of high thermal efficiency, which achieves extremely low levels of CO2 emissions. In HCCl, because auto-ignition occurs without external ignition-timing control means, such as spark plugs, the timing of ignition is determined based on complicated auto-ignition process. Thus, in HCCl, it is difficult to control the timing of ignition. Particularly, if engines operated in an HCCl mode with high load, abrupt increase in pressure in their combustion chambers due to knocking or sudden increase in pressure could cause noise. Thus, the operating ranges of known engines in terms of load in the HCCl mode are limited at high load.
One type of known engines with HCCl function is designed to use internal EGR (Exhaust Gas Recirculation). This type of engines controls intake and exhaust valve overlap during which exhaust gas with a high temperature is recirculated via the exhaust port into the combustion chamber, so that stratification of fresh intake air and the recirculated exhaust gas is created in the combustion chamber. Another type of known engines with HCCl function is designed to use external EGR. This type of engines is equipped with an external pipe connecting the exhaust port to the intake port, an example of which is disclosed in Japanese Patent Application Publication No. 2001-214741. An internal combustion engine based on the external EGR is designed to feed exhaust gas with a relatively high temperature back through the external pipe into the combustion chamber via the intake port, so that stratification of fresh intake air and the recirculated exhaust gas is generated in the combustion chamber. In these engines with internal or external EGR, the stratification of intake air and the recirculated exhaust gas facilitates reduction in fuel burning velocity.